Although V12 engines reached their height of use between World War 1 and II in aircraft, they were displaced quickly by the advent of turbo-prop and jet engines. There have been inherent problems for use in vehicles, finding only occasional use in exotic cars, due to their size, complexity and cost. Improvements in combustion chamber design and piston forms enabled lighter, shorter V8 engines to surpass the V12s, starting in the 1930s, and they essentially disappeared after WWII, except for a few top-of-the-line luxury and sports cars, such as those of Rolls-Royce, Jaguar, Mercedes-Benz, BMW, Ferrari, Aston Martin and Lamborghini. V12s were common in Formula One race cars through about 1980, but the Ford Cosworth V8s proved to have better power-to-weight ratios and less fuel consumption, so they became more successful, in spite of being less powerful and having less endurance than the best V12s of that era.
V12 is a common configuration for large diesel engines used in trucks and marine use. In gasoline and diesel-fueled engines, V12 is a common configuration for tank and other armored fighting vehicles.
The firing of cylinders in a 4-stroke engine fall into two main classes: Even Fire and Odd Fire:                Even Fire is when the cylinder fires at or near Top Dead Center (TDC) of the 3rd stroke, so that the firing, including lead time, produces an efficient and rapidly propagating flame front throughout the cylinder in respect of the fuel being burned. The result is development of combustion peak pressures at or very near TDC, thus providing the maximum power stroke travel of the piston.        Odd Fire is when the cylinder firing is delayed well into the 3rd stroke, for example 15-35° after TDC. Depending on the number of cylinders, Odd Fire is required in some V-type engines as a result of the angle between the cylinder banks and the geometry of the firing sequence, that is, where the several pistons are respectively positioned in the 720 degrees of rotation to complete the 4 cycles. Other considerations for the delay include balance and vibration induced by the rotational dynamics of the engine during operation. Of course, Odd Fire reduces the efficiency of an engine. A 30° or so delay robs that cylinder of roughly half its power, so that in an engine having some of the cylinders set for delay to reduce or eliminate the induced vibration, the maximum theoretical power output cannot be reached. Delay can also induce premature ignition knock. The power-to-weight ratio drops, so other cylinder configuration engines may make more sense to use.        
V8s are designed with a 90° V to ensure that a cylinder firing occurs every 90° so that all 8 cylinders have fired in two complete crankshaft revolutions, that is, in the 720° of crankshaft rotation in a 4-cycle engine.
The angle between cylinders has a huge effect on engine compartment layout and center of gravity. Briefly, the wider the angle, the lower the CG. Engine compartment volume requirements directly affect the body configurations, especially in front-engine vehicles, which is critical for good aerodynamics, a major contributor to good fuel efficiency.
A conventional Even Fire V12 requires a 60° angle between the two banks of 6 cylinders (60°V-angle). If a V12 has a different V-angle in the block, such as a 90°V, then that configuration requires an Odd Fire timing condition, where some or all of the cylinders do not have combustion peak pressures at or very near top dead center (TDC). Thus, Odd Fire V12s typically do not produce full theoretical power, combustion is incomplete, and the power-to-weight ratio is reduced. In addition, the 90°V configuration produces vibrations in a 12 cylinder engine that are not present in an 8 cylinder engine, again due to the rotational dynamics described above. To resolve the vibration problem, the angle is narrowed to 60°.
Thus, V12s have not gained acceptance because they are stuck between two limiting choices: 1) To use a 90°V, you must have Odd Fire with the result of loss of power and performance on the one hand, and if you try Even Fire, you get rough, induced vibration operation; 2) On the other hand if you use a 60°V, you raise CG, increase aerodynamic drag, and engines are more costly to make, not fitting within the manufacturing processes for V8s.
Accordingly, there is an unmet need in the art to provide an improved V12 engine that more nearly achieves the potential advantages of that size and type of engine: namely, greater power-to-weight ratio, lower CG than a 60°V-angle between banks, improved engine compartment layout, adaptability to all types of fuels and all fields of engine use, smooth operation through the RPM curve, better RPM curve shift points, greater torque, greater overall power, slower running for improved engine life, lower cost per cubic inch displacement, and ease of production for engine constructors set up for conventional V8-type engine production.